feat(core): implement data retention, metrics, and enhanced media handling

This commit introduces several significant improvements across the backend and gateway services:

- **Data Retention**: Added an automated cleanup scheduler in `discord-gateway` to prune expired messages, attachments, and voice recordings based on configurable retention policies.
- **Observability**: Integrated `prom-client` in the `backend` service to expose Prometheus metrics via `/api/metrics` and added default Node.js runtime metrics.
- **Media Handling**: Enhanced `MediaHandler` in `discord-gateway` to support media URL resolution and improved playback status tracking.
- **API & Config**: Expanded the configuration endpoint to expose more system settings and reorganized `.env.example` for better readability.
- **Refactoring & Cleanup**:
    - Removed unused `better-sqlite3` dependency.
    - Refactored voice channel routing.
    - Improved error handling and testing coverage with comprehensive unit tests for shared utilities and error classes.
- **Documentation**: Added `MEMORY.md` for project context.

services/backend/src/modules/config/config.routes.ts

@@ -9,6 +9,18 @@ export function createConfigRouter(): Router {
   router.get("/config", (_req, res) => {
     res.json({
       monitorGuildId: config.MONITOR_GUILD_ID || null,
+      webserverPort: config.WEBSERVER_PORT,
+      nodeEnv: config.NODE_ENV,
+      backlogSyncHours: config.BACKLOG_SYNC_HOURS,
+      backlogSyncBatchSize: config.BACKLOG_SYNC_BATCH_SIZE,
+      retentionMessagesDays: config.RETENTION_MESSAGES_DAYS,
+      retentionAttachmentsDays: config.RETENTION_ATTACHMENTS_DAYS,
+      retentionVoiceDays: config.RETENTION_VOICE_DAYS,
+      autoDeleteFlaggedEnabled: config.AUTO_DELETE_FLAGGED_ENABLED,
+      aiAnalysisEnabled: config.AI_ANALYSIS_ENABLED,
+      voiceGuildId: config.VOICE_GUILD_ID || null,
+      voiceChannelId: config.VOICE_CHANNEL_ID || null,
+      logLevel: config.LOG_LEVEL,
     });
   });